1. Field of the Invention
The present invention relates to a method for dynamically modulating the driving current of a backlight module, and more specifically to a method for tuning the driving current of a backlight module according to the integral gradation of a frame.
2. Description of the Related Art
The display generated by a conventional cathode ray tube (CRT) is an impulse-type image that enables the viewer to see a high contrast frame. In contrast, when a liquid crystal display (LCD) shows a frame, its backlight source maintains a constant brightness, while the varying orientation of the liquid crystal molecules allows the rays emitted from the backlight source to go through in order to create many gradations or gray levels. Generally, the display generated by a CRT has a color contrast superior to the one generated by an LCD, and the contrast ratios (CR) from the measurement of these frames can further support the fact. The contrast ratio is expressed as follows:
      CR    =                  L        W                    L        B              ;wherein LW is the measured brightness of a frame when a pixel displays a true white, and LB is the measured brightness of a frame when a pixel displays a true black.
The black display of a CRT is a true black image; hence its contrast ratio is higher than an LCD's. On the other hand, the display generated by an LCD is a hold-type image. The black display of an LCD is not a true black image due to the constant luminance of the backlight source, hence the image of an LCD is not sufficiently sharp and bright in its color contrast.
FIG. 1 is a functional block diagram of a conventional liquid crystal display. A timing controller 13 generates timing signals that trigger a scanning driver 12 and a data driver 17 to be active to send the LCD panel 11 various driving signals. Furthermore, a backlight module 14 is placed behind the LCD panel 11 as a light source to highlight the frame displayed by the LCD panel 11 for the viewer to watch. Presently most backlight modules 14 employ a cold cathode fluorescent lamp (CCFL) as a light source, hence a DC/AC converter 15 is necessary to supply AC power to the CCFL.
Regarding conventional LCDs, the CCFL is mostly powered by a constant current source; hence a backlight source always steadily luminesces behind the LCD even though all of the pixels of a frame display black. The steady existence of the backlight source results in a lesser contrast ratio for the LCD. Therefore, in order to improve the contrast ratio of the LCD, several methods are put forth for effectively shading the rays from the backlight source during a true black frame as far as possible. However, these methods cannot effectively shade the emitted rays completely so the contrast ratio is still inferior.
In summary, LCDs with a superior contrast ratio are needed for the display market to provide viewers with a better image quality.